The present invention relates to a technique effectively applied to an oscillation circuit using an external part such as an oscillator like a quartz oscillator or an electronic part for oscillation like an oscillation module. More specifically, the invention relates to a technique effectively applied to an oscillation circuit for generating a reference frequency signal of a semiconductor integrated circuit device for high frequency which demodulates a reception signal and modulates a transmission signal in a wireless communication system such as a portable telephone.
In a semiconductor integrated circuit for high frequency (hereinbelow, called an RF IC) for modulating a transmission signal and demodulating a reception signal, a PLL (Phase Locked Loop) circuit having a VCO (Voltage Controlled Oscillator) for generating an oscillation signal of a predetermined frequency to be synthesized with a reception signal or a transmission signal is used. As it is known, the PLL circuit is a circuit for comparing an oscillation output of the VCO with a signal of a predetermined frequency as a reference by a frequency comparator (also called a phase comparator) and feedback controlling the VCO so that the phase frequencies of the two signals coincide with each other.
Since relatively high precision is required for the oscillation frequency of the VCO used for a portable telephone, it is difficult to use, as a reference oscillation circuit, a circuit which can be provided on a chip, such as an LC oscillator or a ring oscillator. Generally, an oscillation circuit using an external part such as an oscillator like a quartz oscillator or an electronic part for oscillation like an oscillation module is employed.